FIG. 1 shows a conventional TRIAC dimmer 10 including resistors R1 and R2, a capacitor C1, a bidirectional trigger diode (DIAC) 12, and a TRIAC switch 14. The resistor R1 adopts a variable resistor. The resistors R1 and R2 and the capacitor C1 are serially coupled between two terminals of an alternating current (AC) power 16. The TRIAC switch 14 has a first terminal 142 and a second terminal 144 coupled to the two terminals of the AC power 16, respectively. A third terminal 146 of the TRIAC switch 14 is coupled to the capacitor C1 via the DIAC 12. At the beginning, the TRIAC switch 14 is in an off state. Namely, the AC voltage Vac is not input to a load. Moreover, the resistors R1 and R2 generate a current according to the AC voltage Vac for charging the capacitor C1. When the voltage at the capacitor C1 reaches a breakover voltage of the DIAC 12, the DIAC 12 will be turned on, thereby turning on the TRIAC switch 14. When the TRIAC switch 14 is turned on, the AC voltage Vac is input to the load and the capacitor C1 starts discharging. The TRIAC switch 14 keeps on until the AC voltage becomes zero or until a holding current I1 that passes the TRIAC switch 14 is lower than a threshold. That is to say, the TRIAC dimmer 10 converts the AC voltage Vac into an AC phase-cut voltage Vtr with a conduction angle to the load, as shown by a waveform 20 of the AC voltage Vac and a waveform 22 of the AC phase-cut voltage Vtr in FIG. 2. The conduction angle of the AC phase-cut voltage Vtr can be controlled by controlling a resistance value of the resistor R1. Namely, a conduction time Tc and a non-conduction time Tnc of the AC phase-cut voltage Vtr can be controlled. When the resistance value of the resistor R1 increases, the conduction angle of the AC phase-cut voltage Vtr decreases. Namely, the conduction time Tc of the AC phase-cut voltage Vtr decreases. Oppositely, when the resistance value of the resistor R1 decreases, the conduction angle of the AC phase-cut voltage Vtr increases. Namely, the conduction time Tc of the AC phase-cut voltage Vtr increases.
FIG. 3 shows a LED driver 30 with the TRIAC dimmer 10. The TRIAC dimmer 10 receives the AC voltage Vac and outputs the AC phase-cut voltage Vtr with the adjustable conduction angle. A rectifier 32 rectifies the AC phase-cut voltage Vtr to generate a DC phase-cut voltage Vin. Resistors R3 and R4 divide the DC phase-cut voltage to generate a voltage Vd, thereby allowing an integrated circuit (IC) 34 to acquire information of the DC phase-cut voltage Vin. The IC 34 controls the switching of the transistor Q1, thereby controlling a current at the LED string 36, so that the illumination of the LED in the LED string 36 can be controlled. However, as shown by FIG. 1, during the conduction of the TRIAC switch 14 of the TRIAC dimmer 10, the holding current I1 will be generated. However, the holding current I1 easily causes an abnormal waveform of the DC phase-cut voltage Vin, thereby resulting in a flickering of the LED string 36. In order to avoid the flickering, a bleeding circuit (not shown) is generally utilized for generating a bleeding current to countervail the influence of the holding current I1 on the DC phase-cut voltage Vin. Moreover, the holding current I1 relates to a time proportion D that is determined by the conduction time Tc or the non-conduction time Tnc of the DC phase-cut voltage Vin and a cycle T. Accordingly, a bleeding signal that is related to the time proportion D is required for controlling the bleeding current. Wherein, the time proportion D equals Tc/T or Tnc/T. FIG. 4 shows a conventional method for acquiring the bleeding signal Vdut. In the IC 34, a voltage-to-time circuit 38 generates signals Sd and Sdn according to the voltage Vd that is related to the DC phase-cut voltage Vin. Wherein, the signal Sdn is an inversion signal of the signal Sd. Referring to FIG. 5, the voltage-to-time circuit 38 can be implemented by a comparator 42. The comparator 42 compares the voltage Vd with a preset reference voltage Vref so as to generate the signal Sd. The signal Sd and the voltage Vd as well as the DC phase-cut voltage Vin have the same cycle T. Selecting the proper reference voltage Vref allows a pulse width of the signal Sd to equal a conduction time Tc of the voltage Vd, as shown by a waveform 44 of the voltage Vd and a waveform 46 of the signal Sd in FIG. 5, and consequently the signal Sd contains the information of the time proportion D=Tc/T. Referring to FIG. 4 again, the switching of switches SW1 and SW2 controlled by the signals Sd and Sdn generates a voltage Vh which has the same time proportion D=Tc/T as that of the signal Sd. An RC filter 40 formed by a resistor Rrc and a capacitor Crc filters the voltage Vh to generate the bleeding signal Vdut. Wherein, the bleeding signal Vdut is an average value of the voltage Vh, so the bleeding signal Vdut contains the information of the time proportion D. Other circuits in the IC 34 control the bleeding current according to the bleeding signal Vdut, thereby preventing the LED string 36 from flickering.
However, a frequency of the AC voltage Vac varies between 40 Hz and 60 Hz. Namely, the capacitor Crc with a large capacitance value is required for generating a larger RC time constant. Accordingly, the conventional circuit needs to increase a pin to connect the external capacitor Crc. Obviously, the conventional method for acquiring the bleeding signal Vdut is not suitable for the IC with low pin numbers. Therefore, it is desired a circuit and a method without extra pins to acquire the bleeding signal Vdut.